1. Field of the Invention
The present invention relates to a vacuum degassing apparatus for molten glass which removes bubbles from molten glass continuously supplied.
2. Discussion of Background
In order to improve the quality of formed glass products, there has been used a vacuum degassing apparatus which removes bubbles generated in molten glass before the molten glass which has been molten in a melting furnace is formed by a forming apparatus. Such a conventional vacuum degassing apparatus is shown in FIG. 12.
The vacuum degassing apparatus 200 shown in FIG. 12 is used in a process wherein molten glass C in a melting tank 212 is vacuum-degassed and is continuously supplied to a successive treating vessel (not shown), e.g. a treating vessel for plate glass such as a floating bath and an operating vessel for bottles. A vacuum housing 202 where a vacuum is created has a vacuum degassing vessel 204 substantially horizontally housed therein, and an uprising pipe 206 and a downfalling pipe 208 housed in both ends thereof so as to extend vertically and downwardly. The uprising pipe 206 has a lower end immersed in the molten glass G in an upstream pit 214 which communicates with the melting tank 212. The downfalling pipe 208 also has a lower end immersed in the molten glass G in a downstream pit 216 which communicates with the successive treating vessel (not shown).
The uprising pipe 206 communicates with the vacuum degassing vessel 204. The molten glass G before degassing is drawn up from the melting tank 212 into the vacuum degassing vessel 204. The downfalling pipe 208 communicates with the vacuum degassing vessel 204. The molten glass G after degassing is drawn down from the vacuum degassing vessel 204 and is led to the successive treating vessel (not shown). In the vacuum housing 202, thermal insulation material 210 such as bricks for thermal insulation is provided around the vacuum degassing vessel 204, the uprising pipe 206 and the downfalling pipe 208 to cover these parts for thermal insulation. The vacuum housing 202 may be made of metal such as stainless steel. The vacuum housing is evacuated by a vacuum pump (not shown) to maintain the inside of the vacuum degassing vessel 204 therein in a depressurized state such as a pressure of 1/20-1/3 atmosphere. As a result, the molten glass G before degassing in the upstream pit 214 is sucked up by the uprising pipe 206 to be introduced into the vacuum degassing vessel 204. After the molten glass is vacuum-degassed in the vacuum degassing vessel 204, the molten glass is withdrawn down by the downfalling pipe 208 to be led into the downstream pit 216.
In the conventional vacuum degassing apparatus 200, the molten glass G that has a high temperature such as a temperature between 1200-1400.degree. C. is treated. In order to deal with such a high temperature treatment, portions in direct contact with the molten glass G such as the vacuum degassing vessel 204, the uprising pipe 206 and the downfalling pipe 208 are constituted by circular shells which are normally made of noble metal such as platinum, and platinum-rhodium and platinum-palladium as platinum alloy, as disclosed in JP-A-2221129 in the name of the applicants. The applicants have used circular shells of platinum alloy for these members to put the vacuum degassing apparatus into practice.
The reason why these members are constituted by the circular shells made of noble metal such as platinum alloy is that not only the molten glass G is at a high temperature but also a low reactivity of the noble metal with the molten glass at a high temperature prevents the molten glass from being made to be heterogeneous by reaction with the molten glass, that there is no possibility of mixing impurities into the molten glass G, and that required strength can be ensured to some degree at a high temperature. In particular, the reason why the vacuum degassing vessel 204 is constituted by a circular shell made of noble metal is that the circular shell is self-heated by flowing an electric current in the circular shell per se, and the molten glass G in the shell is uniformly heated to maintain the temperature of the molten glass G at a certain temperature, in addition to the reasons as just stated.
When the vacuum degassing vessel 204 is made of noble metal, a circular shell is appropriate in terms of mechanical strength such as high temperature strength. Since noble metal such as platinum is too expensive to increase the wall thickness, the circular shell has a limited diameter and can not be formed in a large size because of both of cost and strength. This has created a problem in that the vacuum degassing apparatus can not be formed so as to have a large quantity of flow because of the presence of a limited quantity of flow of the molten glass G which can be degassed by the vacuum degassing vessel 204. If the vacuum degassing vessel 204 in a circular shell has the entire length thereof extended and the current of the molten glass is increased to make the volume larger so as to increase a degassing throughput, there has been created a problem in that the apparatus is extended and cost is raised. That is to say, there has been created in a problem in that the degassing throughput (the quantity of flow) of the molten glass G in the vacuum degassing apparatus can not be made large.
Since the molten glass G is obtained by dissolution reaction of powdered raw materials, it is preferable that the temperature in the melting vessel 212 is high in terms of dissolution and that the viscosity of the molten glass is low or the temperature of the molten glass is high in terms of vacuum-degassing. Although the conventional vacuum degassing apparatus 200 requires to use alloy made of noble metal in the vacuum degassing vessel 204 and the like in terms of high temperature strength, it is difficult to increase the wall thickness of the circular shells in terms of cost because such alloy is expensive. Even if noble metal such as platinum is used, the temperature of the molten glass at an inlet of the vacuum degassing apparatus 200 has been limited to a certain temperature (1200-1400.degree. C.) as stated earlier.
The appropriate temperature at which a forming machine (forming treatment vessel) forms the molten glass after degassing has been limited to a certain temperature though the temperature varies depending on articles to be formed, such as plate glass and bottles to be formed. When noble metal is used to form the vacuum degassing vessel 204, the temperature of the molten glass G at the inlet of the vacuum degassing apparatus 200 has been restricted to a temperature lower than 1400.degree. C. This has created a problem in that a drop in temperature of the molten glass G in the vacuum degassing apparatus 200 decreases the temperature of the molten glass G at an outlet of the vacuum degassing apparatus 200 to a lower temperature than the temperature required for forming since the quantity of flow (throughput) can not be made greater and the quantity of heat carried in by the molten glass G is not so large. This requires that the molten glass G in the vacuum degassing vessel 204 be uniformly heated as stated earlier. For this uniform heating, the vacuum degassing vessel 204 per se is required to be constituted by a circular shell made of noble metal, causing the problem in that it is difficult to increase the throughput as stated earlier.
In order to cope with these problems, a proposal has been made to use inexpensive refractory material such as firebricks in paths of the vacuum degassing vessel 204, the uprising pipe 206 and the downfalling pipe 208 instead of using expensive noble metal material such as platinum alloy.
There has been known a bubble forming phenomenon that use of refractory material in the melting furnace generates fine bubbles from the surface of the refractory at an initial stage when the refractory used as the refractory material directly contacts with the molten glass. These bubbles are classified into two kinds of bubbles, one kind of the bubbles that are generated as carbon dioxide (CO.sub.2) gas or nitrogen (N.sub.2) gas by combination of oxygen with carbon, carbide or nitride as impurities on a surface of the refractory because of the presence of a reduced state on the surface of the refractory in touch with the molten glass, and the other kind of the bubbles that are generated from the surface of the refractory because gas in the pores in the refractory contacts with the molten glass.
In general, as the pores in the refractory, there are open pores (apparent pores) that open to the outer surface of the refractory and closed pores that do not open to the outer surface and exist independently. When the vacuum degassing apparatus 200 is prepared from a refractory which has at least one kind of pores, the gas included in the pores promptly becomes bubbles at initial contact with the molten glass and a small amount of bubbles are generated from the pores after that in the case of the open pores. In the case of closed pores, the gas included in the pores is prevented from promptly becoming bubbles at the initial contact between the refractory and the molten glass. However, it is supposed that the surface of the refractory is gradually worm out by erosion and the closed pores in the refractory accordingly contact with the molten glass to gradually make bubbles originating from the gas included in the pores.
When the refractory material is used for a path in the vacuum degassing apparatus 200, there is a possibility that bubbles are intermittently generated from the refractory material for a long period of time even after commencement of operation.
When the refractory material is used for a path in the vacuum degassing apparatus, the temperature of the molten glass G is proposed to be set at about 1200-1400.degree. C. in order to avoid a change in the conventional vacuum-degassing treatment conditions with platinum used as the refractory material and to prevent the erosion of the refractory material from being accelerated by increasing the temperature of the molten glass to a high temperature. This degassing treatment temperature, (about 1200-1400.degree. C.) is relatively lower than the temperature in the conventional degassing treatment process with only a refiner used, that is to say, the degassing treatment temperature, (about 1400-1500.degree. C.) in a process wherein bubbles are grown by the refiner, and the bubbles are rising in the molten glass to be eventually broken on the liquid surface of the molten glass for degassing. It is supposed that, in the vacuum degassing apparatus with the refractory material used in the path for the molten glass, the erosion rate of the refractory material used in the path is so small that the closed pores in the refractory are scarcely exposed on the surface of the refractory to generate bubbles.
However, the vacuum degassing apparatus with refractory material used in the path has created a problem in that the viscosity of the molten glass G is high in comparison with the degassing treatment with the refiner used since the degassing treatment is carried out at a lower temperature than the degassing treatment temperature in the conventional refining step with only the refiner used, and that the bubbles which have generated on the surface of the refractory have too small a rising speed to sweep a possibility of insufficient degassing.
If the vacuum degassing vessel 204, the uprising pipe 206 and the downfalling pipe 208 are built from more inexpensive refractory material than noble alloy, and if the molten glass can be continuously vacuum-degassed as in the case of use of noble alloy, it would not necessary to restrict material use in terms of cost or to restrict the size in terms of decreased strength caused by the restriction of the material use in comparison with the case of use of noble alloy such as platinum. Design freedom would be remarkably improved not only to become capable of construct the vacuum degassing apparatus in a large quantity of flow but also to become capable of vacuum-degassing at a higher temperature.
However, if all constituent parts in the vacuum degassing apparatus 200 are built from firebricks, the following problem are raised. Since nothing supports a pipe-like open end such as the lower end of the uprising pipe 206 or the downfalling pipe 208, it is necessary to support heavy firebricks only by the adhesive power of bond, which is difficult to obtain sufficient strength. If the firebricks are prepared in a long cylindrical shape, the cost is remarkably increased. Under the circumstances, it is practically difficult to build the lower end of the uprising pipe 206 or the downfalling pipe 208 from firebricks.
Even if the lower end of the uprising pipe 206 or the downfalling pipe 208 is built from firebricks, there are created problems in that damage or deterioration is likely to cause at joints between firebricks, and that the firebricks are reactive and likely to be selectively deteriorated at a position in the vicinity of an interface between the molten glass G and air in the melting vessel 212 because of presence of a high temperature and the air at that position. The deterioration at the joints or the interface could deform the lower end of the uprising pipe 206 or the downfalling pipe 208 in an unequal shape in the direction of height, damage such as breakage could be caused, and the lower end of the uprising pipe 206 or the downfalling pipe 208 could be partly broken and fallen out, causing a problem in that sufficient durability can not be obtained. If broken firebricks are mixed into the molten glass G, there is caused a problem in that it is impossible to hold homogeneous composition in the glass.
When the path for the molten glass having a high temperature is made of platinum as in the conventional vacuum degassing apparatus, the formation of holes due to wearing of the thin platinum must be taken into account at designing, and the apparatus is required to enable repair and replacement of platinum for a short period of time after the production of glass products has been temporary standstill. For repair and replacement of the path, it has been necessary to release the reduced state and expel all the molten glass from the inside of the vacuum vessel, the uprising pipe and the downfalling pipe, to drop the temperature of the entire vacuum apparatus to an ordinary temperature, and then to carry out repair or replacement of platinum. Since it is appropriate that the molten glass is cut at the lower ends of the uprising pipe and the downfalling pipe for repair or replacement of platinum, the vacuum degassing apparatus has been required to have a structure that the entire apparatus can be lifted by at least 1 m to separate the uprising pipe and the downfalling pipe from the high temperature glass reservoirs thereunder when the uprising pipe and the downfalling pipe are repaired. However, lifting the entire vacuum degassing apparatus 200 having a solid structure has required an extremely difficult operation accompanied by danger since the apparatus is large and extremely heavy and the apparatus is put under the reduced state at a high temperature during operation.
As stated earlier, the paths of the molten glass such as the vacuum degassing vessel 204, the uprising pipe 206 and the downfalling pipe 208 that directly contact with the molten glass G have been made of platinum or platinum alloy such as platinum-rhodium in the conventional vacuum degassing apparatus 200. Although noble metal or its alloy such as platinum or platinum alloy is good at resistance to high temperature and high temperature strength in comparison with other metal, noble metal or its alloy has inherent limitations. In order to make the vacuum degassing vessel 204, the uprising pipe 206 and the downfalling pipe 208 larger, it is necessary to make the wall thickness of the vessel and the pipes thicker. However, noble alloy such as platinum is extremely expensive, and the production of the vacuum degassing apparatus 200 becomes remarkably costly. The cost of the apparatus tremendously increases because a larger size of the apparatus requires thicker wall for the vessel and the pipes. In the case of use of noble alloy metal, there are limitations to enlargement of the apparatus in terms of cost.
This has created a problem in that it is impossible to build the vacuum degassing apparatus so as to have a large quantity of flow because of the limitations in the quantity of flow of the molten glass G which can be degassed in the vacuum degassing vessel 204.
As explained, the conventional vacuum degassing apparatus is costly in construction and can not be built so as to provide a large quantity of flow though the degassing efficiency of the molten glass is extremely high. As a result, the conventional vacuum degassing apparatus has been mainly used for glass which has a specialized application such as an optical use and an electronic use wherein the presence of fine bubbles is not acceptable, and which is produced in small-quantity production.
As stated earlier, it is preferable that the temperature in the melting vessel is high when glass is molten in the melting vessel, and it is also preferable that the temperature of the melting vessel is high when the vacuum degassing treatment is carried out. Even if noble metal such as platinum is used, the strength necessarily becomes lower as the temperature becomes higher. An increase in the wall thickness of the vacuum degassing vessel directly contributes to an increase in cost. Under the circumstances, the temperature of the molten glass at the inlet of the vacuum degassing apparatus has been limited to 1200-1400.degree. C., and has been enable to be raised to a desired temperature.
On the other hand, it has recently been required that a vacuum degassing apparatus having a high degassing efficiency is used to carry out a degassing treatment for mass-quantity production of glass such as glass for construction or automobiles. As the vacuum degassing vessel, the uprising pipe and the downfalling pipe are prepared from noble metal alloy such as platinum, it is not acceptable in terms of cost to use the conventional vacuum degassing apparatus with noble metal used therein for preparation of glass to be produced in mass production since noble metal such as platinum is extremely high.
If the vacuum degassing vessel 204, the uprising pipe 206 and the downfalling pipe 208 are constituted by refractory material in the conventional vacuum degassing apparatus 200 shown in FIG. 12 to try to make the apparatus larger and to make the degassing throughput larger, there is created a problem in that refractory material bubbles generate into the molten glass from the surface of the refractory material.
When the vacuum degassing vessel 204, the uprising pipe 26 and the downfalling pipe 208 of the vacuum degassing apparatus 200 are constituted by refractory material, this creates problems in that the presence of joints between pieces of refractory material involves deterioration the joints by the molten glass G having a high temperature, and that the lower ends of the uprising pipe 206 and the downfalling pipe 208 are subjected to deterioration at an interface with air as a free surface of the molten glass G since the lower ends are immersed in the upstream and downstream pits 214 and 216. When the vacuum degassing apparatus is constituted by refractory material, the apparatus is heavier as a whole than the vacuum degassing apparatus mainly constituted by platinum because the structure of the refractory material is dense and the used refractory material is mainly electro-cast bricks. It is an extremely difficult and dangerous operation to lift the vacuum degassing vessel 204, the uprising pipe 206, the downfalling pipe 208 and the thermal insulation material 210 in the vacuum housing 202 as a whole as in the conventional vacuum degassing apparatus 200 shown in FIG. 12.
When a vacuum degassing apparatus having a small degassing throughput (quantity of flow) such as the conventional vacuum degassing apparatus is operated in a single use, the range of available quantity of flow is narrow for adjustment in the quantity of flow of the molten glass G in response to required production of glass, creating a problem in that it is difficult to promptly cope with a change in production.
If the platinum or the platinum alloy that forms the vacuum degassing vessel 204, the uprising pipe 206, the downfalling pipe 208 and so on is broken, it takes several months to repair it, creating a problem in that the unavailability of the vacuum degassing apparatus during repairing of it prevents glass products from being produced.